1. Technical Field
This disclosure relates generally to two dimensional (2D) electronic materials. Specifically, the disclosure pertains to the synthesis and patterning of 2D materials in controlled configurations for devices.
2. Related Art
2D nanostructure materials are the latest in a long line of useful materials that hold great promise for high-density electronics. Because of their nanoscale dimensions, electrons become quantum confined within and imbue these 2D nanostructures with unique new properties. Zero dimensional (0D) nanostructure materials such as quantum dots exhibit new luminescence and absorption bands as sizes become small. One dimensional (1D) nanostructure materials such as carbon nanotubes and semiconductor nanowires, for example, of Si, are known as quasi-1D materials with excellent conductivity (metallic) or semiconducting properties. Other nanostructure materials, such as boron nitride are excellent insulators. However, wiring such 0D and 1D nanostructure materials to fabricate integrated circuits may present significant problems.
2D materials with nanoscale thickness, for example, graphene, boron nitride, metal chalcogenides and others, are produced from layered materials (e.g. graphite, BN and MoS2) that have been reduced to one, or a few atoms, thick. These nanoscale thickness materials are highly desirable, often yielding new properties. Graphene may remain metallic in a single-layer form, such that semiconducting properties needed to make transistors are missing in graphene. However, metal chalcogenides are mostly semiconducting, and come in a great variety. 2D nanostructure material families may be synthesized, and may be utilized in metals, insulators, and semiconductors. The beauty of these materials is that they have quantum properties, and are essentially thin films—very thin, but nevertheless a thin film.
Chemical vapor deposition (CVD) growth poses a challenge when attempting to make 2D crystals of electronic-grade monolayers or few layered semiconductors with large areas. For graphene, millimeter sized hexagonal or square crystals may be grown. For the metal chalcogenides, typically a few to 10-20 microns, or 100 micron-sized triangular crystals may be grown. The field follows the synthesis of graphene, which followed the synthesis of nanotubes. Tube furnaces are typically used with vaporized reactants, which self-assemble on various substrates. For graphene, the substrates are metals at 500-1000 C. For metal chalcogenides, the substrates are typically oxides at ˜500-900 C. In both cases, the crystals are sometimes attached epitaxially and covalently to the substrate, but in most cases the crystals float on the substrates held only by (strong) van der Waals forces. These crystals may be removed by dissolving the substrate. Then the crystals (held onto a piece of polymer film) may be stamped on top of one another, and the polymer may then be dissolved away. In this way, vertical junctions may be made between two semiconductors, or a metal (graphene) and semiconductor, etc.
For semiconductors, p-n junctions may be used to make diodes and gated transistors, etcetera. For example, a p-n junction may comprise p-doped semiconductor and an n-doped semiconductor, or junctions may comprise two different p-type or different n-type semiconductors. However, it is difficult to grow two types of semiconductors together, laterally, to make multiple heterojunctions, which may be utilized in applications such as quantum wells. Repeating growth steps may pose a problem since each material has an optimum growth temperature, and at higher temperatures, CVD growth may evaporate away materials, for example, metal chalcogenides may lose their chalcogen atoms.